Thomasset's frog is the largest of the Seychelles’ native frogs, reaching a maximum length of 55 mm. Much information is lacking about this species, although it is known to be restricted to relatively undisturbed habitat in wet, rocky areas along streams, and has a preference for higher altitudes. This species is a rock climber, and is most often found at night sitting on rocks and large boulders. It is the rarest frog species in the Seychelles and is currently threatened by habitat degradation, mainly due to fire and invasive species. It might be at risk because of its small range, which makes it vulnerable to threats such as ecological disasters or climate change.

Urgent Conservation Actions

Long-term monitoring of this species; sustainable development of ecologically friendly tourism should be encouraged.

This species was names in honour of Mr. H. P. Thomasset, who collected the first formal scientific specimen of this species in 1907.

Thomasset's frog is the largest of the Seychelles frogs, reaching a maximum total length of 55 mm in females. This species is still very small relative to the world’s largest frog, the Goliath frog of West Africa, which can reach body lengths of 400 mm and weigh 3.1 kg.

Before my visit my mind’s eye had Seychelles down as one of the tropical Indian Ocean paradises that has sadly ended up in the whirling, slightly out of co...
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Evolutionary Distinctiveness

Order: Anura

Family: Sooglossidae

Oceanic islands are typically lacking in native amphibians, because very few species have any tolerance to salt water as a result of their sensitive skin, which is often used as a respiratory surface. The Seychelles is the only island group with an endemic family of frogs, i.e. one where all of the member species occur in the same island group and nowhere else on earth. This family is called the Sooglossidae (or sooglossids), and is commonly referred to as the “Seychelles frogs”. Although it is thought that amphibians may occasionally make the perilous journey across seas and oceans via rafting on mats of vegetation, being carried by strong air currents, or human introductions, the Seychelles frogs were transported to their current location by continental drift over millions of years. Around 200 million years ago at the beginning of the Jurassic period, the Seychelles islands were joined to the eastern part of the ancient southern supercontinent Gonwana, which went on to split apart into Australia, Antarctica, India, Madagascar and the Seychelles over millenia of movement of the earth’s plates. The sooglossids survived the geological breakup of India and the Seychelles around 65 million years ago, which is around the same time as the dinosaurs became extinct, and the closest known relatives of the Seychelles frogs are today found in India.

The sooglossids include some of the world’s smallest frogs, with adult Gardiner’s Seychelles frogs measuring just 10-11 mm in total length. Their closest living relative is the purple frog (EDGE rank 4) in the Western Ghats, India, which was only formally discovered in 2003 because it spends most of the year buried up to 4 metres underground. These lineages of amphibians may have been more diverse on Indo-Madagascar in the Cretaceous period, but now they are reduced to two miniscule ancient families that are consequently highly evolutionarily distinct. It is thought that the two lineages diverged around 131 million years ago in the Early Cretaceous period, which is over 30 million years before humans and elephants shared a common ancestor. The slow migration and fragmentation of the Indo-Madagascan fragment of Gondwana subsequently brought each landmass to its current position, separating the Seychelles frogs and the purple frog by about 2,500 km of Indian Ocean.

The Seychelles comprise eight main granitic islands and over 80 tiny islands and coral atolls. The group of islands is 1,000 km across at its widest point and the total land area is around 455 km sq., with the highest point being Morne Seychellois on the granitic island of Mahé, which 914 metres above sea level. The distribution of amphibian species across the Seychelles has probably been impacted by changing sea levels over the millennia. The central granite Seychelles islands were once a continuous landmass of around 130,000 km sq. called the Seychelles Bank, which has been reduced over time by erosion and rising sea levels. It was reduced to its present condition of scattered islands about 10,000 years ago. As a result, there is little endemism between islands since they were all connected relatively recently.

Today, the Seychelles frogs are only present on the larger granitic islands of Mahé and Silhouette. In total, the Seychelles contains 13 species of amphibians – 6 caecilians and 7 frogs, all of which are endemic apart from the Mascarene grassland frog (Ptychadena mascareniensis), which is widespread across sub-Saharan Africa, the Mascarene Islands and Madagascar. All the amphibian species are confined to the granitic islands of the Seychelles, with the larger islands having more species than the smaller islands. They are all ground dwelling, reproducing independently of water. They exhibit a high degree of parental care, with egg guarding occurring in all four species, and parents carrying tadpoles on their back until metamorphosis in one species, the Seychelles frog. Despite being very similar in external appearance, the Seychelles frogs may be identified by their advertisement calls, which are all highly distinctive.

Description

Thomasset's frog is the largest of the Seychelles frogs. Fully grown females measure about 55 mm in total length, with males being slightly smaller at a total length of 45 mm. This species has large, golden eyes and a fairly pointed snout. The colouration of the back is golden to reddish brown, with some scattered black markings. A few individuals have a broad, brown stripe running down the centre of the back. Black bars are present across the toes and legs, and many individuals also have a thin white or yellow line down the middle of the back and across the backs of the thighs. Most individuals also have a white line behind each eye which terminates just above the fore-limbs. There is a double row of 2-6 small, light-coloured bumps along the back, which begin behind each eye. The ventral or under surfaces are brown with some light mottling patterns. The tips of the toes are slightly expanded, and are thought to provide increased friction during climbing.

Ecology

Thomasset's frog is a rock climber, and is most often found at night sitting on rocks and large boulders. It may also sometimes be encountered climbing on low branches of trees and scrubs. The call of this species is similar to that of the Seychelles frog (Sooglossus sechellensis), but the notes are produced at a slower rate. The call is longer and the first guttural note is repeated 3 or 4 times, rather than just once. The entire song has been described as sounding like: "rraack-rraack-rraack-toc-toc-toc-toc". Nothing is known about the feeding ecology of Thomasset's frog.

The mode of development and level of parental care is not confirmed for this species. However, the eggs of this Thomasset's frog are thought to exhibit direct development (hatching into froglets rather than tadpoles) in moist nests on land among rocks, with females guarding the eggs until they hatch. This assumption is based on the observation that females produce large, yolky eggs (a typical feature of direct developing eggs), and also that no tadpoles of this species have ever been found in streams or small ponds within this species' range in the Seychelles. The prediction of female eggs guarding is derived from the fact that females of close relatives of Thomasset’s frog (Gardiner's Seychelles frog and the Seychelles frog) are known to guard their eggs.

Habitat

This species is restricted to relatively undisturbed habitat in wet, rocky areas along streams or dry streambeds. Thomasset's frog has recently been recorded in low altitude secondary forest, but generally shows a strong preference for higher altitudes and undisturbed habitats.

Distribution

Thomasset's frog is restricted to Mahé and Silhouette Islands in the Seychelles, occurring at relatively high elevations above 350 metres above sea level, although one individual was recently found at 95 metres above sea level. The area of occupancy of this species is probably less than 20 km sq..

Thomasset's frog is the rarest of the Seychelles frogs (and in fact the rarest frog in the Seychelles), but there are currently no population estimates for this species.

Population Trend

The current population trend of Thomasset's frog is specified as unknown by the IUCN Red List of Threatened Species.

Status

Thomasset's frog is listed as Vulnerable in the IUCN Red List of Threatened Species because it is known from only two locations, with a total area of occupancy of less than 20 km sq..

Threats

The main threat to this species is habitat degradation, mainly due to fire and invasive species, but it is not thought to be seriously threatened in its higher altitude habitat. It might inherently be at risk because of its small range, which makes it vulnerable to threats such as ecological disasters or climate change.

Conservation Underway

Thomasset's frog occurs within the 30.5 km2 Morne Seychellois National Park in Mahé, whilst its Silhouette distribution falls partly in the site of a conservation project that has been established on the island by the Nature Protection Trust of the Seychelles (an NGO with coordinates conservation work on Silhouette) via their Islands Development Company Silhouette Conservation Project. However, the forests are not legally protected. In addition, many small reserves on the islands have been set up by the Seychelles Government and independent agencies to protect specific species and general habitat areas.

The challenges facing conservation projects are very different on the islands of Mahé and Silhouette, the former being a much larger island (148 km2) with a population of 70,000 and number land use conflicts, and the latter being small (just 20 km2) with a population of only 150 people. Silhouette appears to support healthy populations of all four species of Seychelles frog with no immediate threats. However, future hotel and infrastructure development might be a potential threat to these species. In contrast, significant tracts of potentially suitable habitat in Mahé’s central and southern mountains lack any protection against encroaching development, and the distribution and status of Seychelles frogs in these areas is completely unknown.

Nature Seychelles (another local conservation NGO in the Seychelles) recently initiated a project to investigate the status of these frogs on Mahé, supported by the Royal Society for Protection of Birds (RSPB) and the Herpetological Conservation Trust in the U.K. This project tested methods for monitoring Seychelles frog populations and made recommendations regarding the most practical system for determining distribution and numbers of frogs in the field. It also outlined a long-term monitoring programme for regular assessment of the status and abundance of the Seychelles frogs, and identified priority research needs. Following this work, Nature Seychelles is implementing a programme to look at the long-term status of these frogs. This will enable the population trend of this species to be determined, which is essential for appropriate conservation actions to be made.

The long-term monitoring of Thomasset's Seychelles frog is a vital priority to ensure its conservation in the wild, allowing for any population declines to be quickly detected and investigated. Further research should be conducted into the threat processes impacting this species in Mahé’s central and southern mountains, and a Conservation Action Plan should be developed to address the conservation of Thomasset's Seychelles frog in this region. The development of Mahé and Silhouette for the tourism industry and local population increase is a significant threat to the Seychelles’ biodiversity, including the Seychelles frogs. Wherever possible, sustainable development of ecologically friendly tourism should be encouraged. The Seychelles is marketed as a tourist destination on the basis of its natural beauty, so strategic development of eco-tourism could be immensely beneficial to the conservation of the Seychelles’ endemic amphibians.